The present invention relates to a secondary battery using a non-aqueous solvent, more particularly to a secondary battery using a non-aqueous solvent which is less in self-discharge, has good charge/discharge cycle characteristics and is excellent in storability.
In recent years, as a secondary battery which is compact and lightweight and has high energy density, those which is used a kind of a conductive polymer as a negative electrode material and the negative electrode is combined with an electrolyte dissolved an alkali metal ion such as lithium ion, sodium ion and the like have been attracted attention. This kind of secondary battery is doping/undoping or intercalating/deintercalating the aforesaid metal ion electrochemically to the negative electrode and this doping/undoping phenomenon, etc. are utilized as a charge/discharge step. Thus, it has an advantage that, as caused in the secondary battery which is employed a conventional alkaline metal itself as the negative electrode, internal short due to precipitation of the alkaline metal dendritely during the discharge or remarkable deterioration of charge/discharge efficiency can be eliminated.
As this kind of secondary battery, there has been known one, for example, in which a negative electrode comprising a polyacetylene and a lithium ion are combined as disclosed in Japanese Provisional Patent Publication No. 136469/1981. Further, as a conductive polymer which is usable for a negative electrode material, there may be mentioned, in addition to the above polyacetylene, a linear high molecular compound having a conjugated double bond such as a poly(p-phenylene), a polypyrrol, a polythienylene, a polyaniline, a poly(p-phenylenesulfide), a poly(p-phenyleneoxide) and the like.
Since these conductive polymer is lightweight and a potential for doping/undoping an alkali metal ion such as lithium ion, etc. to said polymer is substantially similar as a charge/discharge potential in case that the alkali metal is employed as a negative electrode, it has an advantage that an energy density per unit weight is high when it has used in a secondary battery. However, the aforesaid conductive polymer is unstable at a state of doping the alkali metal ion, that is, a charged state, and since it reacts with a solvent or decomposes itself, there are problems that a self-discharge becomes remarkably great as well as a cycle characteristic will be deteriorated.
On the other hand, there has been reported a secondary battery in which a graphite having a structure of spreading conjugated double bonds quadratically is used as a carbonaceous material and a graphite interlayer compound which is intercalated by electrochemically reducing an alkali metal ion is employed as a negative electrode active substance. However, in such a secondary battery, since an alkali metal-graphite interlayer compound formed by charging is chemically unstable and it reacts with solvent in accordance with destruction of a graphite structure, there are disadvantages that its storability is bad and deterioration of charge/discharge efficiency as well as lowering of cycle characteristics will be occurred.
Further, there has been reported a secondary battery in which, as the carbonaceous material for constituting a negative electrode, those obtained by carbonizing an organic high molecular compound such as a phenol resin, a polyacrylonitrile, a cellulose and the like have been used. For example, in Japanese Provisional Patent Publication No. 209864/1983, a secondary battery using as a negative electrode a carbonaceous material obtained by heat treating an aromatic condensed polymer, which has an atomic ratio of hydrogen/carbon being in the range of 0.15 to 0.33 has been disclosed.
Such a secondary battery has high output power as compared with a secondary battery using the conventional conductive polymer or a graphite as a negative electrode. However, it is the actual circumstances that there is not yet improved at all in the point that the negative electrode reacts with an electrolytic solvent irreversibly during charged state so that an increase of self-discharge and a deterioration of cycle characteristics will be caused.
As stated above, there remains a problem that a negative electrode material which utilizes a dope/undope or an intercalate/deintercalate phenomenon of an alkali metal ion is used in a battery, in all cases, a self-discharge increases and a cycle lifetime is short.